Simple Drivers Make High-Power LED Lighting More Cost-Effective

Light-emitting diodes ( LEDs ) have become the mainstream for streetlights and other high-power lighting applications due to their power-saving and maintenance-free features. Although there are many ways to drive LED lighting, a new simple topology circuit to drive multiple LED strings is necessary to meet the market's demand for higher efficiency and lower system costs. Looking at streetlights or high-bay lights for stadiums and other high-power lighting applications, the trend is gradually turning to solid-state lighting using light-emitting diodes (LEDs) as the light source. The main reasons are that LEDs have higher energy efficiency and less frequent maintenance requirements, which also prove that such a shift is indeed necessary. In such high-power lighting applications, various methods are currently being considered to drive these lights. This article will discuss a new topology that can drive multiple LED strings with higher efficiency and lower system cost. To fully understand the advantages of this topology, we will first explore various methods that are currently being considered solutions or have worked well in low-power LED applications. Driving Multiple LED Strings for Efficiency Parallel/Series Solutions Have Their Pros and Cons The simplest way to drive multiple LED strings is to use a power supply that converts the mains voltage to a direct current (DC) output voltage (e.g., 12 volts or 24 volts) and then use this power supply to drive parallel LED strings, using resistors in each string to regulate the current. This method is very low cost, but today's high-brightness LEDs can consume more than 350 milliamps (mA), so this method has very high losses, resulting in low efficiency and poor current regulation, which makes the light difference between the strings very obvious. To improve this method, linear regulators must be used instead of resistors to improve the consistency of light output of all the strings. However, this only makes the light output consistent, without significantly improving efficiency or power consumption. Reducing power consumption is very important to maximize the life of the LED. In both methods, using resistors or linear regulators as a fixed heat source will greatly shorten the life of the LED. Another equally simple method is to make a long single series string and use a single power supply that can generate a high-voltage DC stable current source. This approach operates at high voltages above the 60VDC or 42V RMS safety extra low voltage (SELV) level, and the lighting fixture or accessories must be approved by safety agencies, which greatly reduces the flexibility of using the same motor design for other applications. Another consideration for the single string approach is reliability. If only one LED is turned on, the light output of the entire lighting fixture is released. Although many arc suppression circuits or devices can be added to control the turning on of each LED, this will increase the cost and complexity of the lamp.

Light-emitting diodes (LEDs) have become the mainstream for streetlights and other high-power lighting applications due to their power-saving and maintenance-free features. Although there are many ways to drive LED lighting, a new simple topology circuit to drive multiple LED strings is necessary to meet the market's demand for higher efficiency and lower system costs.

Looking at the development trend of streetlights or high-bay lights for stadiums and other high-power lighting applications, the trend is gradually turning to solid-state lighting using light-emitting diodes (LEDs) as the light source. The main reasons are that LEDs have higher energy efficiency and less frequent maintenance requirements, which also prove that such a shift is indeed necessary.

In such high-power lighting applications, various methods are currently being considered to drive these lights. This article will discuss a new topology that can drive multiple LED strings with higher efficiency and lower system cost. To fully understand the advantages of this topology, we will first explore various methods that are currently being considered solutions or have worked well in low-power LED applications.

Driving Multiple LED Strings for Efficiency Parallel/Series Solutions Have Their Pros and Cons

The simplest way to drive multiple LED strings is to use a power supply that converts the mains voltage to a direct current (DC) output voltage (e.g., 12 volts or 24 volts) and then use this power supply to drive parallel LED strings, using resistors in each string to regulate the current. This method is very low cost, but today's high-brightness LEDs can consume more than 350 milliamps (mA), so this method has very high losses, resulting in low efficiency and poor current regulation, which makes the light difference between the strings very obvious.

To improve this method, linear regulators must be used instead of resistors to improve the consistency of light output of all the strings. However, this only makes the light output consistent, without significantly improving efficiency or power consumption. Reducing power consumption is very important to maximize the life of the LED. In both methods, using resistors or linear regulators as a fixed heat source will greatly shorten the life of the LED.

Another equally simple method is to make a long single series string and use a single power supply that can generate a high-voltage DC stable current source. This approach operates at high voltages above the 60VDC or 42V RMS safety extra low voltage (SELV) level, and the lighting fixture or accessories must be approved by safety agencies, which greatly reduces the flexibility of using the same motor design for other applications. Another

consideration for the single string approach is reliability. If only one LED is turned on, the light output of the entire lighting fixture is released. Although many arc suppression circuits or devices can be added to control the turning on of each LED, this will increase the cost and complexity of the lamp.

Using buck converters to regulate current is key to reducing driver circuit cost

In fact, in high-power LED lighting applications, the most common architecture is a multi-string architecture that uses switching regulators to regulate current, where a single main power supply converts the alternating current (AC) power to a single DC bus voltage that is generally below the SELV level; this bus then powers parallel LED strings, each of which has a buck converter (most common) or a boost converter. For simplicity, the analysis in this article is limited to buck converters, which are very similar to boost converters in terms of cost and component count. For

example, Figure 1 shows a low-cost, simple buck regulator circuit that includes a pulse width modulation (PWM) controller, an inductor, a metal oxide semiconductor field effect transistor (MOSFET), a diode, and multiple resistors and capacitors. If higher efficiency is required, a MOSFET can be used instead of a diode, and a PWM controller capable of synchronous buck operation can be used.

Figure 1 Simple buck regulator

Figure 2 shows the subsystem blocks for a high-power multi-string lighting application using a buck regulator for current regulation. Once the AC input is rectified, it is fed to a power factor correction (PFC) boost circuit, where the PFC generates a high voltage of 400 volts to provide input power to a downstream isolated DC-DC converter. The converter output is then used to generate a low voltage bus (typically 12 or 24 volts) to power the buck-regulated LED strings.

Figure 2. Typical high-power LED lighting system using a buck regulator

This method has high efficiency and is ideal for LED lighting with a minimum number of LED strings. However, for high-power applications with more than four strings, the number of components and costs will increase. For electronic component manufacturers and supply chains, product sales will increase; however, for lighting equipment manufacturers and their users, such high costs are not conducive to the widespread use of products, because the stable development of solid-state lighting depends on low-cost drive circuits to allow the market to take shape and grow steadily.

Low-cost/high-efficiency electrical isolation design solution is outstanding

Figure 3 shows a series input multiple parallel LED simple driver, which is a very cost-effective way to drive multiple LED strings. In addition to PFC, this is also a two-stage method, including a reverse-regulated current buck regulator and a downstream DC-DC converter circuit. This method is very efficient and has excellent string current regulation function, and most importantly, it is a low-cost method.

Figure 3 Simple drive multi-transformer

此外，针对各个灯串加装的单一被动硅控制整流器(SCR)消弧电路，这个方法也能够达到备援效用。如果一个LED或灯串开启，则光线输出不会高于其他灯串。

在深入研究其中的运作之前，必须先讨论对于使用简易驱动多变压器方法时出现的问题。首先要注意这是电气绝缘设计，其中可设计二次侧输出电压维持在SELV位准以下，便不须让照明设备与电源结合与互连，以获得安全机构的许可。原因与本文讨论的所有脱机解决方案一样，电源仍然须要安全许可，但是灯具并不需要，便省去一道流程。

此外，将输出维持在这些位准以下，亦可增加本身的弹性，使各种灯具都能满足其他许多照明应用的需求。

另从散热管理的角度来看，这种绝缘设计较为理想，因为其中没有对LED近接或接触金属附件的任何限制。更显著的特点是，这种绝缘设计不需输出端的回授，故不必使用光电或其他安全额定的绝缘回授装置，所以，本文也会探讨二次侧的简易性，因为二次侧只有少数的被动组件，且没有任何偏压电源、主动组件或操控装置。

总结来说，在运作方面，简易驱动器拥有1%以上的绝佳灯串电流匹配，而且具有高效率的谐振运作，能够随着灯串数增加而达到更高的成本效益。

PFC circuit output reduces switching losses

Next, the output of the PFC circuit is discussed. It is the input mode of the reverse buck circuit and can be configured to generate a stable current output. The system closed loop is located near this current. Therefore, the generated current output is supplied downstream to the DC-DC transformer circuit, which includes a half-bridge controller, two MOSFETs, capacitors C1 and C2, and multiple transformers.

This current then flows through the half-bridge MOSFET switches to the primary side of the series transformer, where capacitors C1 and C2 perform many functions, not only to create a voltage divider for the half-bridge, but also as a component of the resonant circuit and a DC blocking capacitor to help avoid transformer saturation. The resonant operation allows the MOSFET switches to switch with zero voltage switching (ZVS), which reduces switching losses and forces the output diode to achieve zero current switching (ZCS) for maximum efficiency.

It is important to note that the DC current, now converted to AC current, will resonate back and forth through the primaries of all the series connected transformers. The number of transformer primaries that can be connected in series is quite flexible because the winding turns ratio can be chosen to support many transformers or LED strings. However, the calculation of the turns ratio must take into account the number of strings, as it dictates the number of transformers and the forward voltage of each string.

PFC design considerations are essential to maximize power conversion efficiency

To maximize the efficiency of power conversion, the least amount of power must be processed, which can be achieved by operating as close to the input voltage as possible. Since most high-power lighting applications support the use of active PFC, for simplicity it can be considered as a functional block and some typical outputs are used to represent the outputs.

Since most active PFC circuits function as boost converters, the PFC output voltage must be set higher than the peak of the highest AC line voltage. In the typical input range of 85 to 265 VAC, this is about 375 volts. After adding some dynamic range for margin and tolerance, 400 volts becomes a typical setting.

In addition, to ensure that the downstream buck has more dynamic range of PFC output changes, more tolerance must be added to accommodate the ripple of about 40 volts, which makes the lower limit of the reverse buck input operating point about 360 volts. In addition, to ensure that the buck output has a certain maximum voltage drop in order to operate normally, a certain dynamic range must also be provided, limiting the output range to 280 volts.

Calculating the step-down/transformer turns ratio to stabilize the current value is coming soon

After understanding the limits of each range, the next step is to understand how to calculate the stable current value of this design example through the step-down voltage and transformer turns ratio. In this design, two transformers are used to drive four LED light strings with a current of 1 ampere (A). Each light string has ten high-power LEDs. Assuming the LED forward voltage is 3.5 volts and the light string voltage is 35 volts, since the output operating point of the DC step-down is set at 280 volts, it becomes the input of the DC-DC transformer circuit at this time.

This means that the voltage applied to the series primary will be half the voltage across the capacitor divider (composed of C1 and C2), resulting in a series primary configuration voltage of 140 volts. This is shown in Equation 1:

The calculation of turns ratio becomes quite easy through the equation of Formula 1, where NP = number of primary turns; NS = number of secondary turns; VS = secondary or LED string voltage; VP = voltage across each primary winding.

On the other hand, if one wants to calculate the current output setting value for reverse buck when each transformer drives two LED light strings, one must first confirm that only one light string of each transformer is conducting in the alternating half cycle.

However, to maintain the LED conductive state during the dormant period, the current provided to the conductive light string must be twice the LED current; that is, when the required LED current is 1 amp, the current provided to the LED and filter capacitor is 2 amps per half cycle. To calculate the buck regulator, the current value (ISet) must be set as shown in Equation 2:

As mentioned above, determining the transformer requirements is fairly simple, making the simple driver a flexible solution for many different lighting applications. However, for the simple driver to be part of a block approach for many LED lighting applications, it is also necessary to consider the upstream power stages, such as the power handling components of the half-bridge, flyback buck and PFC, as these power stages must be scaled to handle the maximum power level that the driver is expected to achieve.